It is known that, in the synthesis of methyl tert.butyl ether (MTBE), dimethyl ether (DME) is formed as a by-product, depending on the reaction conditions. This dimethyl ether remains in the product raffinate after washing with water and distillation (if the reaction mixture is washed with water). For the further processing, different requirements are made on this raffinate. In some cases higher DME contents may be tolerated. In other processes (e.g. the HF-alkylation) too high a DME content may cause noticeable trouble. Therefore, if the raffinate is intended for further processing, as in HF-alkylation, a product purity of max. 10 to 15 ppm of DME in the raffinate is required.
In the prior art processes, the DME content in the raffinate may be as high as 100-300 ppm. To remove the DME from the raffinate, repeated stripping of the raffinate is necessary (see Chemistry and Industry, Aug. 21, 1982, page 570 ff). Since this raffinate, if starting from a C.sub.4 -hydrocarbon cut from a steam cracker or an FCC unit, often contains besides the C.sub.4 -components also noticeable amounts of propane and propylene (3 to 10%), the separation of DME is elaborate and expensive because DME forms with propane an azeotrope containing 8% DME.
In the synthesis of methyl tert.-amyl ether (TAME), dimethyl ether (DME) is formed as a by-product as well.
In general, alcohol to isoolefine mole ratios of 0.5-6:1, preferably 1-2:1, are recommended for the etherification of isobutene and isoamylene, respectively, with methanol. According to the prior art, the reaction temperature may be 50.degree. C.-120.degree. C., preferably 70.degree. C.-120.degree. C., and the appropriate reaction pressure is 5-50 bar; see for instance DE-OS 29 11 077, pages 4 and 5, and European Patent Application 0015 513, page 6. It is also known that the first reaction stages may be provided with circuits for removing the reaction heat, see SRI-Report 76-1-1, page 5/6.
In U.S. Pat. No. 4,262,146 it is suggested to split up the hydrocarbon stream in order to control the temperature; depending on the temperature rise in the first reactor, part of the isoolefine component by-passes the first reactor and is introduced into the second reactor.
In none of the known publications is the formation of DME considered. Attention is rather paid to procedures for separating DME from the product stream.
It is an object of this invention to perform the MTBE synthesis and the TAME synthesis such that the DME formation is suppressed to the extent that subsequent removal of DME from the raffinate can be omitted. Other objects will be apparent to those skilled in the art.